"The primary determinants of Earth's climate are based on orbital-astronomical cycles of 21,000-, 40,000-, and 100,000-year duration... The key idea is that the amount of solar radiation received at the Earth's surface varies, not only with the seasons, but with these longer-term variations in the Earth's orbit. Over time, it was shown that the major changes in climate of past epochs correlated with these orbital cycles."

See: Laurence Hecht article (2006-04)